The present invention is directed to high speed semiconductor lasers. More particularly, this invention is directed to a method for increasing the bandwidth in InGaAsp lasers, said method comprising doping the active layer of said lasers, p-type, e.g., with Zn, Cd, Be, Mg, and the like.
InGaAsp lasers are reliable light sources, especially suitable for high data rate optical communication systems. High data rates are generally limited by the devices intrinsic bandwidth performance. Previous techniques applied to increase laser modulation bandwidths are:
(a) use of short cavity lengths. PA1 (b) Operation of the laser at high bias optical power in analog applications.
Both of these techniques are useful for increasing modulation bandwidths, but each has technical limitations. The shortest practical limit to the cavity length is about 50 .mu.m which is due to the technical difficulties of cleaving and wire bonding to a short cavity device. Moreover, it is generally undesirable to bias the laser at high bias power because of increased system noise and reduced device reliability.
Previous experimental work has shown that direct amplitude modulation bandwidths of 4 to 5 GHz are possible in a number of AlGaAs laser structures, see Figueroa et al., IEEE J. Quantum Elect., QE-18, 1718 (1982). More recently, X-band small signal modulation bandwidths of AlGaAs lasers of 11 GHz have been achieved by use of a short cavity window laser biased at 16 mw/facet of optical power, see Lau et al., Postdeadline Paper WJ1, Conf. Opt. Fiber Conn., (1984).
Small signal experiments of 1.3 .mu.m wavelength Double channel planar-buried heterostructure (DCP-BH) lasers with a resonance frequency as high as 9.3 GHz have been reported, see Tucker et al., Electron. Lett., 20: 393 (1984). However, the modulation amplitude of the DCP-BH laser exhibits a very strong roll-off at frequencies well below the resonance frequency.
It is known that short cavity vapor phase regrown--buried heterostructure (VPR-BH) lasers with a p-doping of 2.times.10.sup.18 cm.sup.-3 have a wide modulation bandwidth and a flat optical response. See, Su et al., Appl. Phys. Lett., 45: 1302 (1984) and Su et al., Appl. Phys. Lett., 46: 344 (1985). A flat optical response is obtained for the VPR-BH lasers grown on conductive substrates rather than on semi-insulating substrates. At a bias optical power of only 6.9 mW/facet, the bandwidth is 12.5 GHz. The limited bandwidth of the detector prevents measurement of the frequency response at higher bandwidths.